The present invention relates in general to the field of program code conversion. In particular, the invention relates to a method and apparatus for emulation of condition code flags.
In the field of computer processing, it is often desired to run program code written for a computer processor of a first type (a “subject” processor) on a processor of a second type (a “target” processor). Here, an emulator is used to perform program code translation, such that the subject program is able to run on the target processor. The emulator provides a virtual environment, as if the subject program were running natively on a subject processor, by emulating the subject processor.
Most conventional computer processors comprise a set of registers, which are a small number of high-speed memory locations closely associated with the processor. The registers can be contrasted with general random access memory provided remote from the processor, which is slower but has a much larger capacity. Typically, a modern processor includes of the order of 32 to 144 registers. The limited size and high speed of the registers make them one of the most critical resources in almost all computer architectures.
The registers are usually augmented with a set of condition code flags. The condition code flags are set or cleared such as in response to the execution of instructions in the processor and are used to represent the outcome of various operations. The value of one or more of the condition code flags is often used as the basis for a decision during execution of a sequence of instructions. Hence, in the context of program code conversion, it is critical to accurately emulate the condition code flags.
Whilst there tends to be some overlap between the set of condition code flags of a subject processor and the set of condition code flags of a target processor, in almost all cases there is not an exact correspondence between the two sets of flags. Therefore, accurately emulating the subject condition code flags can be an expensive operation. The degree to which flag emulation impacts upon the efficiency of the conversion process varies according to compatibility of the subject processor architecture with the target processor architecture, but in general the emulation of subject condition code flags amounts to a significant proportion of overall performance.
A further problem arises in that some processor architectures are inherently highly incompatible. Some processors are strongly reliant on the use of condition code flags, whereas some other processors do not provide any condition code flags at all. For example, it is very expensive to provide an emulator able to convert subject code written for a subject processor which does use condition code flags to run on a target processor without any condition code flags.